File:Loopy star trails show the effect of Euclid's Fine Guidance Sensor intermittently losing its guide stars ESA25121347.jpg

English: Euclid’s Fine Guidance Sensor (FGS) is a completely new development in Europe, and it is responsible for ensuring the mission points with precision, performing all the ‘slews’ (rotations) that a six-year survey mission requires.
The FGS is an onboard instrument equipped with optical sensors that image the sky from the sides of the ‘field of view’ of Euclid’s VISible instrument (VIS). The sensor uses guide stars to navigate and feeds this data into the spacecraft's Attitude and Orbit Control System to orient and maintain the telescope's precise pointing.
Before launch, the Sensor was rigorously tested, but nothing compares to the true sky under real space conditions. Cosmic rays – high energy radiation originating from the Universe and from solar flares from our Sun – sometimes caused ‘artefacts’ or false signals to appear in Euclid's observations. These false signals intermittently outnumbered real stars and Euclid's Sensor failed to resolve star patterns that it needed to navigate. This led to some interesting test results!
The most ‘loopy’ show an extreme case of Euclid failing to lock into place while observing a star field, resulting in an image of swirling star trails and ‘lassos’ as the spacecraft tried to home in on its target. Clearly, to reveal hard-to-see, subtle patterns in distant galaxies and star clusters, this won’t do. Teams got to work to come up with a fix.
The software patch was tested first on Earth with an electric model of Euclid and a simulator, then for ten days in orbit. The signs were positive, as more and more stars revealed themselves.
“Our industrial partners – Thales Alenia Space and Leonardo – went back to the drawing board and revised the way the Fine Guidance Sensor identifies stars. After a major effort and in record time, we were provided with new on-board software to be installed on the spacecraft,” explains Micha Schmidt, Euclid Spacecraft Operations Manager.
“We carefully tested the software update step by step under real flight conditions, with realistic input from the Science Operations Centre for observation targets, and finally the go-ahead was given to re-start the Performance Verification phase.”
Giuseppe Racca, Euclid Project Manager adds; “The performance verification phase that was interrupted in August has now fully restarted and all the observations are carried out correctly. This phase will last until late November, but we are confident that the mission performance will prove to be outstanding and the regular scientific survey observations can start thereafter.”